Osteoclasts are unique cells of hematopoietic origin, which possess the capacity to resorb bone. However, the lineages that give rise to osteoclasts are not well described. In preliminary data we found that a highly purified population of murine bone marrow cells expressing the surface marker, CD45R/B220, produced osteoclast-like cells (OCL) after stimulation with M-CSF and RANKL. CD45R1B220 is an antigen that is most often expressed on cells of the B-lymphocyte lineage although it may also be expressed on more immature cells. We have found that ovariectomy increased the number of OCL that formed in cultures of CD45R/B22O+ about bone marrow cells. Examination of RANK expression in bone marrow cells showed that relatively few cells express this protein on their surface. However, treatment with M-CSF in vitro induced expression of RANK mRNA in bone marrow cell cultures. Furthermore, we found that interleukin-l type 1 receptor deficient (IL-1R1-/-) mice, which fail to lose bone mass after ovariectomy, also do not increase the number of OCL that form from CD45R/B220F bone marrow cells after ovariectomy. In addition, in both wild type and IL-1R1-/- mice ovariectomy had no effect on the number of OCL that formed in CD45R1B220- about bone marrow cell cultures. These results imply that estrogen mediates its effects on osteoclast precursor cell number in bone marrow through changes in the CD45R/B220+ osteoclast precursor cell population and that expression of RANK is a critical late event in osteoclastogenesis. The studies of this grant are designed to 1) determine the phenotype of CD45R/B22O+ osteoclast progenitors and their role in ovariectomy induced bone loss and 2) examine the role that RANK expression has in osteoclast formation from CD45R/B220+ about cells.